1. Field of the Invention
The present invention relates to a magnetic field shield including a superconductive film which shields magnetic fields utilizing superconductivity. 2. Prior Art
As a magnetic field shield utilizing superconductivity, the first class superconductor or the second class superconductor has been used depending on the intensity of a magnetic field. The magnetic field shield using the first class superconductor utilizes perfect diamagnetism (Meissner effect), a characteristic of superconductivity. This magnetic field shield cannot shield intense magnetic fields since its critical magnetic flux density is low. The magnetic field shield using the second class superconductor utilizes a mixture of the superconduction state and the normal conduction state, and its critical magnetic field is separated into the upper and lower critical magnetic fields. Since the intensity of the upper critical magnetic field is extremely high, the magnetic field shield using the second class superconductor can be used to shield intense magnetic fields.
With such a magnetic field shield using the second class superconductor, what is called "electromagnetic shielding effect" based on the interlinkage magnetic flux unchangeability principle can also be used to shield intense magnetic fields.
As the above-mentioned magnetic field shield becomes thicker, its shielding effect increases. Therefore, thicker superconductors are used to shield intense magnetic fields. However, a thick superconductor is heated by local magnetic flux flow, and its shielding effect reduces remarkably (secondary harmful effects). When shielding intense magnetic fields using the above-mentioned second class superconductor, relatively thick superconductive sheets or tapes are laminated in layers or laminated with aluminum or copper layers. These two types are inevitably thick and weighty. Thus they are not proper for practical use. In addition, the former type is apt to cause magnetic flux skip (magnetic fluxes enter the shield and move to the center of the shield, raising the internal temperature and allowing more magnetic fluxes to enter the shield, and resulting in a catastrophic phenomenon). The former type thus lacks stability required for shielding magnetic fields. The latter type is better than the former type in structure since the cooling effect due to aluminum or copper layers is added. However, this structure is not enough to fully deliver the characteristics of superconductive material. As this kind of magnetic filed shield is thicker, it produces a greater magnetic field shielding effect (the thickness of the superconductor is said to be proportional to the magnetic field shielding effect). Accordingly, instead of laminating layers of thin superconductive films, it has been considered to be advantageous to use a superconductor which should be as thick as possible, provided that any means for preventing heat generation described above is taken, since production cost and the number of processes can be reduced.